1. Field of the Invention
The present invention relates to an active matrix display device.
2. Description of the Related Art
An electroluminescence element (hereinafter referred to as EL element) is a light-emitting element for emitting light when a current is injected thereto. In an active matrix EL display device, EL elements are arranged in a matrix to form pixels, and pixel circuits are provided for supplying currents to the EL elements of the respective pixels.
The pixel circuits are controlled by scanning lines and signal lines. Each scanning line is commonly provided to pixel circuits arranged in a respective row. Through the connection to those pixel circuits, a signal for selecting the pixel circuit in every row is applied. The signal lines are connected to pixel circuits arranged in a respective column, and a signal corresponding to image information is applied.
US Patent Laid-Open No. 2004/0066357 proposes a pixel circuit in which two signal lines including a signal line for supplying a current signal and a signal line for supplying a voltage signal are provided.
There are two types for arrangement of the pixels, which are a stripe arrangement and a delta arrangement. According to the stripe arrangement, pixels are linearly arranged. According to the delta arrangement, three pixels of RGB which constitute a color display unit are arranged in a delta shape. In a small size display device whose pixel number is small, the delta arrangement is used for the pixel array in many cases for improving definition.
FIG. 6 shows an example of the delta arrangement. In the delta arrangement, pixels of RGB in the row direction are periodically arranged as one sequence, and the pixels adjacent to the pixels in the row direction are shifted in the arrangement sequence by 1.5 pixels.
In a color display device that employs the delta arrangement, an R pixel R1 and a G pixel G1 adjacent to each other in one row forms a pair with a B pixel B1 arranged immediately beneath the row to compose a color display unit. Then, a B pixel B2 adjacent thereto forms a pair with an R pixel R2 and a G pixel G2 immediately beneath the row to compose another color display unit.
In FIG. 6, scanning lines X1, X2, . . . and signal lines Y1, Y2, . . . are also drawn.
In a transmissive liquid crystal device, scanning lines and signal lines are arranged between a pixel and another pixel in order to increase a pixel aperture ratio. In a matrix display device that employs the delta arrangement, it is possible to arrange scanning lines straight through, but it is necessary to thread signal lines among the pixels in a bending manner. Moreover, in order that pixels of the same color are connected to each other by one signal line, connection points C1 and C2 with respect to the pixel circuit are located on the opposite sides in every row.
The connection positions with respect to the signal line are inverted in every row and thus the pixel circuit patterns are arranged so as to be inverted. For this reason, in a precise sense, variations every other row occur in characteristics of TFT elements that compose the pixel circuits. In order to have uniform display characteristics, it is desired to employ a uniform pixel circuit pattern without such inversion.
In a reflective liquid crystal display device or a top emission EL display device, pixel circuits do not block transmitted light, and it is therefore unnecessary to arrange signal lines between pixels and it is also possible to extend the signal lines straight through across the pixel region. However, in the EL display device, the pixel circuit needs to include a few transistors and a power source wiring whose width is large so that a large current flows. For this reason, if the pixel density becomes high, the pixel circuits occupy the entirety of the pixel area. In that case, it is difficult to arrange the signal lines straight through across near the center of the pixel region, and eventually the wiring is bent along a side of the pixel circuit pattern.
U.S. Pat. No. 6,768,482 proposes a top emission EL display device having pixels that are arranged in delta. In this device, a pitch of pixel array in a row direction is set two times larger than a pixel circuit pitch, and instead a pitch of pixel array in a column direction is set half of the pixel circuit pitch, whereby even when the pixels arranged in delta, it is possible to arrange pixel circuits straight through in a stripe manner. Also, it is possible to arrange the signal lines straight through without bending.
However, if the pixel array pitch is further decreased, the pixel circuits need to be arranged at a density two times larger than the degree of decreasing the pixel array pitch. Thus, it is necessary to extremely decrease the sizes of transistors and wirings that compose the pixel circuits. The sizes of the circuit elements and the wiring have lower limits so as to ensure fabrication yield, and setting the pixel circuit pitch smaller than the pixel pitch causes unnecessary disadvantages at the time of pursuing the high definition.